1. Field of the Invention
This invention relates to a process for the production of free-flowing amorphous alkali metal silicates by spray drying, the alkali metal silicate subsequently being impregnated with ingredients of detergents or cleaners, and to the use of the alkali metal silicates thus impregnated in detergents or cleaners.
2. Discussion of Related Art
It is known that hydrated water-soluble particulate silicates still containing about 20% by weight of water can be obtained by spray drying or roller drying of waterglass solutions (cf. Ullmanns Enzyclopadie der technischen Chemie, 4th Edition, 1982, Vol 21, page 412). Such products are commercially available for various purposes. Corresponding powders have a very loose structure as a result of spray drying. Their bulk densities are generally well below 700 g/l, for example 300 g/l or even lower.
Granular alkali metal silicates with relatively high bulk densities can be obtained in accordance with the teaching of European patent application EP-A-0 526 978, in which an alkali metal silicate solution with a solids content of 30 to 53% by weight is introduced into a heated drum in which a shaft comprising a number of arms extending close to the inner wall of the drum rotates about the longitudinal axis, the drum wall having a temperature of 150 to 200.degree. C. and the drying process being supported by a gas introduced into the drum with a temperature of 175 to about 250.degree. C. This process gives a product with an average particle size of 0.2 to 2 mm. A preferred drying gas is heated air.
European patent application EP-A-0 542 131 describes a process in which a product with a bulk density of 500 to 1,200 g/l completely dissolved in water at room temperature is obtained. Heated air is preferably used as the drying gas. This process also uses a cylindrical dryer with a heated wall (160 to 200.degree. C.) in which a rotor with vane-like blades rotates longitudinally at such a speed that a pseudoplastic paste with a free water content of 5 to 12% by weight is formed from the silicate solution with a solids content of 40 to 60% by weight. The drying process is supported by a hot air stream (220 to 260.degree. C.).
Earlier, hitherto unpublished German patent application P 44 19 745.4 also describes a water-soluble, amorphous and granular alkali metal silicate which is produced by a process similar to that described in EP-A-0 526 978, but which does contain silica. The term "amorphous" means "X-ray-amorphous". This means that, in X-ray diffraction patterns, the alkali metal silicates do not produce any sharp reflexes, but at best one or more broad maxima of which the width amounts to several degrees of the diffraction angle. However, this does not mean that regions producing sharp electron diffraction reflexes will not be found in electron diffraction experiments. This may be interpreted to mean that the substance contains microcrystalline regions up to about 20 nm (max. 50 nm) in size.
Granular amorphous sodium silicates obtained by the spray drying of aqueous waterglass solutions, subsequent grinding, compaction and spheronizing with additional drying of the ground material are the subject of U.S. Pat. Nos. 3,912,649, 3,956,467, 3,838,193 and 3,879,527. The products obtained in this way have a water content of about 18 to 20% by weight for bulk densities well below 500 g/l.
Other granular alkali metal silicates with multiple wash cycle performance are known from European patent applications EP-A-0 561 656 and EP-A-0 488 868. The products in question are compounds of alkali metal silicates with certain Q distributions and alkali metal carbonates.
Earlier hitherto unpublished German patent application P 44 46 363.4 describes an amorphous alkali metal silicate with multiple wash cycle performance and a molar Na.sub.2 O:SiO.sub.2 ratio of 1:1.5 to 1:3.3 which is impregnated with ingredients of detergents or cleaners and which has a bulk density of 300 g/l. The silicate carrier particles to be impregnated are preferably present in granular form and/or in the form of a compound with alkali metal carbonates and may be produced by spray drying, granulation and/or compacting, for example roller compacting. In one preferred embodiment, the silicate is impregnated with surfactants, more particularly with nonionic surfactants. By taking up the impregnating agent, the silicate material loses some of its free-flow properties although they can be restored by subsequently treating the impregnated material with an aqueous solution.
However, it has been found that spray-dried amorphous alkali metal silicates free from additional alkali metal carbonates show inadequate flow properties after impregnation and subsequent coating with an aqueous solution.